The invention relates to a steering device for motor vehicles.
Steering devices for motor vehicles transmit a steering intention introduced by way of a steering wheel to a final control element, also called a steering actuator.
In the case of electrically assisted steering devices, power assistance is provided by an electric motor. This may be a power-assisted steering system or a power steering system.
The power assistance may be provided at various points and by various arrangements, depending on the arrangement of the steering device.
In the case of large steering forces and high steering axle loads, the electric motor concentrically surrounds a connecting rod and transmits the power assistance directly to the connecting rod by way of a recirculating ball mechanism. These arrangements are often used in steer-by-wire and rear axle steering systems.
This arrangement has the disadvantage that only small transmission ratios between the driving side and the driven side can be accommodated. Consequently, the electric motor must be capable of delivering a high torque even at low speeds. This can be achieved only by modifying the size of the electric motor accordingly, which means that more overall space must be made available. Due to the immediate proximity to other units, such as the vehicle engine or differential, for example, this additional overall space required can only rarely be found. Moreover, the radiated heat emission from these units may have an adverse effect on the functioning of the steering system, especially the electric motor.
In the case of average demands on the steering device in respect of steering force and steering axle load, the electric motor acts on a drive pinion, which is operatively connected to a connecting rod. In one variant, these steering devices have two drive pinions, one drive pinion being operatively connected to a steering wheel by way of a steering column and a second drive pinion being driven via the electric motor.
These steering devices may offer advantages with regard to the degrees of freedom of the spatial arrangement of the electric motor and the avoidance of conflict with other units. The disadvantage to these steering devices is the fact that the maximum transmissible torque is limited by the drive pinion/connecting rod toothing. In the case of the steering device having two drive pinions, there is the added problem that two toothing systems run together on one connecting rod within a close tolerance zone, in order that they mesh with both drive pinions virtually free of backlash.
The steering device for motor vehicles described in German Published Patent Application No. 198 52 447 has an input shaft, which is operatively connected to a steering wheel by way of a torsionally elastic element. The rotational movement introduced is relayed by way of a planetary gear train to an output member in the form of a drive pinion, and transmitted to a connecting rod. In addition an electric motor acts on the planetary gear train and hence also on the output member by way of a worm gear.
The combination of planetary gear train and worm gear makes this steering device very costly and expensive, since the toothing systems must run free of backlash in order to satisfy the quality requirements with regard to noise and comfort.
This places particularly high demands on toothing quality and support for the components moving in relation to one another.
European Published Patent Application No. 0 870 669 describes a steering device with power-assisting electric motor, which by way of a gear is capable of turning a threaded nut, which by means of a recirculating ball mechanism produces an axial displacement of a connecting rod. At the same time, the recirculating ball mechanism is axially offset in relation to a ball bearing of the threaded nut, which for functional reasons must also have a further bearing for the support in a housing.
U.S. Pat. No. 4,593,780 describes a further steering device with threaded nut, which is likewise supported by two ball bearings.
It is an object of the present invention to make do with one less point of support than in conventional steering devices. This may provide the advantage of providing a steering device which may be used for all different installation variants while at the same time taking up little space and being capable of transmitting a high torque. In addition the electric motor may be flexibly arranged and the steering permits a high transmission ratio from the driving side to the driven side.
The above and other beneficial objects of the present invention are achieved by providing a steering device for motor vehicles as described herein.
This object may be achieved, in particular, in that the steering power assistance generated by an electric motor as a function of a set value predetermined via the steering wheel is transmitted directly to a helical gear by way of a gear. The steering power assistance is transmitted by way of the helical gear to a connecting rod, which is connected to the steered wheels of a motor vehicle. The rotational movement imparted by the electric motor is therefore converted by the helical gear into a translational movement. The helical gear has a recirculating ball mechanism, in order to ensure quiet, functioning with low wear and a simultaneously high efficiency. The helical gear is also capable of transmitting significantly greater forces than a drive pinion/connecting rod combination.
Directly combining the two gear mechanisms produces a direct power flow from the electric motor to the actuator. Much higher transmission ratios may thereby be accommodated than with conventional steering devices, making it possible to reduce the torque that has to be applied by the electric motor. At the same time, this makes it possible to use smaller electric motors, which run at high speeds and have a better efficiency. The tolerance and alignment problems that occur with a drive pinion/connecting rod combination as a result of two toothing systems running together or a steering device with an electric motor, rigidly supported by way of two bearings in a housing and acting on the connecting rod, do not occur in the case of the steering device according to the present invention. On a screw thread side the connecting rod is supported in the housing solely by one bearing. Through suitable choice of the bearing, such as ball bearings, which are capable of bridging an angular position of up to 30 angular minutes, it is possible to compensate for the production tolerances.
The reduced motor torque required means that the gear is less heavily loaded, so that alternative materials may also be used. Such material may be plastic which, due to fatigue strength problems, may be used in conventional arrangements only in a reinforced form.
In one example embodiment, the transmission may be a friction gear, which may provide advantages with regard to efficiency and absence of backlash. Moreover, in the event of overload some slipping is possible, so that an additional safety-friction clutch for overload protection may be eliminated. Another alternative is a gear mechanism with slightly beveled cylindrical gears, in which the backlash compensation may be managed by an adjusting screw or a compression spring.
The principles underlying various exemplary embodiments of the present invention are described below with reference to the Figures.